U.S. Pat. No. 3,420,419 discloses a material holding vessel or tank supported by a frame and wheels which is pulled by a tractor for transportation material over roads or highways. These vessels have a high strength to weight ratio and have special application in the transportation of dry flowable bulk material such as fine granular sand, chemicals, etc. In order to increase profits, the operator will haul a load of one type of dry flowable material to a distant destination and, if possible, return with a load of another type of dry flowable material. This procedure has required the vessel to be washed after the first load is unloaded and before the return load is loaded, which can be a time consuming and costly process. In addition, if there are no washing facilities available, the vessel cannot be washed thereby preventing a return load.